Away from its famous coastline, Sardinia’s rugged interior is home to one of the world’s most intriguing architectural marvels: 7,000 mysterious stone towers known as nuraghi. Built between 1600 and 1000 BC, the circular structures aren’t found anywhere else on the planet, and their origin and purpose remain largely unknown.

Most archaeologists believe that these fortresses were originally built as territorial markers between clans. By climbing one, you could almost always see another, and they formed an ingenious island-wide communication chain. Over time, this early Sardinian society surrounded many of the earlier single-tower structures with protective ramparts, spiral staircases, and as many as 17 connected towers—transforming lone bastions into colossal Bronze Age castles. Today, these otherworldly monuments are still so common throughout the island’s twisting valleys that they’ve come to symbolize Sardinia itself, and the long lost Nuragic civilization that designed them is finally gaining recognition as one of the most advanced in the ancient world.

This year marks the 20th anniversary of Sardinia’s most famous nuragic site, Su Nuraxi di Barumini, being named a UNESCO World Heritage site. To celebrate, “Sardinia’s Stonehenge” is hosting a series of tours and cultural events from June through December, including folk and classical music concerts held inside the site’s megalithic walls. It's an enticing invitation to come face to face with one of Europe’s great mysteries, but with thousands of remarkable nuraghi to choose from, why limit your visit to just one?

The ruins of Nuraghe Prisgiona-Arzachena reveal not only remarkable architectural prowess, but also evidence of a seafaring civilization far ahead of its time.

Alamy

Here are five of the most impressive nuragic sites in Sardinia, along with must-see recommendations nearby for after you get your history fix.

Su Nuraxi–BaruminiClimb 11 towers, thread needle-tight escape routes, and dip into more than 200 homes of a remarkably well preserved prehistoric castle complex that once guarded a surrounding village. Considering that much of humanity was still living in caves and wooden huts when Su Nuraxi was built, its three-story citadels, functioning canal system, and retractable wall entrances are mind-boggling feats of engineering.

Santu Antine–TorralbaSu Nuraxi may be the most famous nuraghi in Sardinia, but Santu Antine is the crowning opus of this mysterious culture. A vast network of passageways made from massive basalt boulders in the 16th century BC leads to spiral staircases that climb 57 feet up to the central spire of a four-tower fortress.

Prisgiona–ArzachenaArtifacts recovered from the sprawling settlement surrounding this three-tower castle show that the nuraghi builders were also one of the most prolific seafaring societies in the Mediterranean, sailing as far as Cyprus. Ancient Sardinians buried their dead in enormous “Giant’s Tomb” grave sites, and Coddu Vecchiu, nearby, is the most impressive and well-preserved on the island.

Nearby, don’t miss: The cerulean seas, puttering yachts, and international jet-set around the dazzling Costa Smeralda. If you splurge on one thing, spend a night at the achingly beautiful Hotel Romazzino, whose whitewashed, bougainvillea-draped facade overlooks an infinity pool and private beach.

The site at Arrubiu-Orroli is the largest nuragic complex in the world.

Alamy

Arrubiu–OrroliThis is the largest nuragic complex in the world, an imposing five-towered fortress built with red-tinted stones. The central bastion has crumbled from 90 feet to 46, and the whole thing was once connected to 12 additional lookout masts by a rampart.

Nearby, don’t miss: The Trenino Verde, an antique steam locomotive and Italy’s most popular tourist train. Pack a copy of D. H. Lawrence’s Sea and Sardinia and retrace the author’s famous jaunt from Mandas to Sorgono by plunging, climbing, and twisting through some of Sardinia’s most stunning—and least explored—landscapes.

Losa–AbbasantaTwo turreted stone walls, a maze of hauntingly lit internal corridors, and steps wrapping around a central tower once made Losa a formidable megalithic fortress. It’s still a site to behold, especially when staring up at the perfectly preserved truncated dome of the 40-foot wide central tower.